The dimensionless coefficients of proportionality f and f . 6.n 

 (9)) probably depend upon the actual conditions at the air-sea inter- 

 face (stage of wave development, wave form, relative wind velocity, 

 stability of the air above the interface, etc.), and therefore they 

 will not be constants. 



Let p = ^/v , and collect the dimensionless factors by 

 putting 



Yon = i ^^^n ^"'^ ^ot = ^ir^S^f^ . 



We then have 



;; 3n _ « ^2 



^n = ± P'^on^^o^ • V^l - V% (10> 



\ = P'Yot^Po^ V Po • ^11) 



The unknown "conditions" of the rough surface are now involved in 

 the dimensionless quantities y and Yq4.» We may call these quan- 

 tities "resistance factors" or "frictional factors." They are given 

 as functions of p = ^/v (or p = ^/v^ resp,), because the hdyro- 

 dynaraic "roughness" of a surface not only depends upon the height 

 of roughness elements (in a geometrical sense), but also upon the 

 steepness d of the waves and their form, which highly determines the 

 roughness conditions of the sea surface. Because d = f(p), and the 

 roughness Involved in f and f. may be expressed as a function of p, 

 it is to be expected that the quantities y and Yq^ are functions 

 of p too (G. Neumann [8]). 



In the formal equations (10) and (11) neither the single resist- 

 ance factors, nor the effective wind velocity v are known with the 

 necessary degree of accuracy. But it seems possible to estimate 

 the effective wind force - the "effective stress" - by means of emp- 



56 



